The walls of hot gas carrying high-temperature gas reactors, e.g. of combustion chambers in gas turbine plants, require thermal protection of their supporting structure against hot gas attack. The thermal protection can be provided, for example, by a hot gas lining in front of the actual combustion chamber wall, e.g. in the form of a ceramic heat shield. A hot gas lining of this kind is generally made up of a number of metal or ceramic heat shield elements lining the surface of the combustion chamber wall. Because of their temperature resistance, corrosion resistance and low thermal conductivity, ceramic materials are ideally suited, compared to metal materials, for constructing a hot gas lining. A ceramic heat shield is described e.g. in EP 0 558 540 B1.
Because of the typical thermal expansion characteristics of the material and the temperature differences occurring during operation—e.g. between ambient temperature when the gas turbine plant is shut down and maximum full-load temperature—flexibility for thermal movement, particularly of ceramic heat shields, as a result of temperature-dependent expansion must be ensured, so that no heat-shield-destroying thermal stresses occur due to said temperature-dependent expansion being prevented. Expansion gaps are therefore provided between the individual heat shield elements in order to allow for thermal expansion of the heat shield elements. For safety reasons, the expansion gaps are designed such that they are never completely closed even at maximum hot gas temperature. It must therefore be ensured that the hot gas does not pass through the expansion gaps to the supporting wall structure of the combustion chamber. In order to seal the expansion gaps against the ingress of hot gas, they are frequently scavenged with seal air flowing in the direction of the combustion chamber interior. Air which is simultaneously employed as cooling air for cooling retaining elements for the heat shield elements is generally used as seal air, which results, among other things, in temperature gradients in the region of the edges of a heat shield element. As a result of the scavenging of the expansion gaps with seal air, the peripheral sides bordering the gaps as well as the cold side of the heat shield elements are cooled. On the other hand, a high heat input because of the hot gas takes place on the hot side of the heat shield elements. Inside a heat shield element, a three-dimensional temperature distribution therefore arises which is characterized by a temperature drop from the hot side to the cold side and by a temperature drop occurring from central points of the heat shield element toward the edges. Therefore, particularly in the case of ceramic heat shield elements, even without contact between adjacent heat shield elements, stresses occur on the hot side which may result in crack initiation and thus adversely affect the service life of the heat shield elements.
The heat shield elements in a gas turbine combustion chamber are typically of flat design and disposed parallel to the supporting structure. A temperature gradient running perpendicular to the surface of the supporting structure only results in comparatively low thermal stresses, as long as unhindered forward flexure in the direction of the interior of the combustion chamber is possible for the ceramic heat shield element in the installed state.
A temperature gradient running parallel to the supporting structure, such as that running from the peripheral surfaces of the heat shield element to the center of the heat shield element, quickly brings about increased thermal stresses because of the rigidity of plate-like geometries in respect of deformations parallel to their largest projection surface. These cause the cold edges of the peripheral surfaces, because of their comparatively low thermal expansion, to be placed under tension by hotter central regions which are subject to greater thermal expansion. If the material strength is exceeded, this tension can result in the initiation of cracks extending out from the edges of heat shield element toward central areas of the heat shield element.
The cracks reduce the load-bearing cross section of the heat shield element. The longer the cracks, the smaller the residual load-bearing cross section of the heat shield element. The thermally induced cracks may lengthen as the result of mechanical stress loads occurring during operation of the gas turbine plant, causing the residual cross section to be reduced still further and possibly necessitating replacement of the heat shield element. Mechanical stress loads of this kind may occur, for example, in the event of oscillatory accelerations of the combustion chamber wall which may be caused by combustion oscillations, i.e. oscillations in the combustion exhaust gases.
In order to reduce the seal air requirement - and therefore thermally induced stresses in heat shield elements, EP 1 302 723 A1 proposes providing flow barriers in the expansion gaps. This can also result in a reduction of the temperature gradient in the region of the edges. However, inserting flow barriers is not always easily possible and also increases the complexity of a heat shield.
In addition, heat shield elements are exposed to severe corrosive attack resulting in a lifetime-limiting loss of material. The material loss occurring in the case of ceramic heat shield elements is attributable to a combination of corrosion, subsequent resintering of the surface and erosive stress caused by the high mass flow of hot gas. Material loss is generally at its greatest where the highest hot gas flow rates obtain. For the ceramic heat shields frequently used nowadays made of corundum and mullite with glass phase, the material loss is essentially due to two reactions, namely first mullite decomposition and secondly grain growth and resintering. The water vapor present in the hot gas results in the decomposition of mullite (3Al2O3*2SiO2 or 2Al2O3*1SiO2) and glass phase to corundum (Al2O3) and silicon oxide (SiOx). The corundum then present at the surface of a heat shield element, both in the matrix of the heat shield element and in the corrosion layer of the mullite grains, exhibits grain growth and sintering. Grain growth and sintering increase with operating time. With increasing numbers of gas turbine startups, this results in a weakening of the surface due to microcracking. Consequently, surface particles are entrained by the high mass flow, resulting in erosion. As a result, the service life of the heat shield elements is limited by corrosion, thereby necessitating premature replacement. Add to this the fact that, in the case of heavy oil operation of a gas turbine, magnesium oxide is added as an inhibitor, which likewise results in corrosive thinning of the heat shield elements. This is caused by the corundum in the heat shield element reacting with the magnesium oxide in the inhibitor to produce spinel as a reaction product. This also results in service life reduction and the need to replace the heat shield element prematurely.
In DE 10 2005 036 394 A1 a material is described in which up to 5 wt % zirconium dioxide powder partially or completely stabilized with magnesium oxide and having a grain size of between 1 and 20 pin, and up to 5 wt % titanium dioxide powder with a grain size of between 50 nm and 20 μm are added to 90 wt % zirconium dioxide free refractory oxide powder with a grain size of between 1 and 150 μm. Up to 5 wt % refractory oxide powder with a grain size of between 1 and 20 μm can also be added to this mixture. Said additional refractory oxide powder is preferably aluminum oxide and/or magnesium oxide and/or yttrium oxide and/or cerium oxide. In the course of sintering above 1550° C. or during use of the ceramic material, the magnesium oxide stabilizer of the zirconium dioxide forms spinel phases and/or magnesium aluminate, and the zirconium dioxide is destabilized. Alternatively or in addition, zirconium titanate and/or aluminum titanate may be formed which, in aggregate, result in subcritical cracking in the ceramic matrix and improve thermal shock resistance. For the most part, only thin-walled, small-volume hollow components can be produced from said material composition, as the sintering is subject to >10 vol % shrinkage.
In CH 469 641 a method for producing a spinel-containing molded element is described. Magnesium oxide is added to an alumino-calcareous silicate glass powder. The mixture is molded and then heated and sintered, the components of the mixture being caused to react and thereby to devitrify by relatively long retention at the sintering temperature of the glass. However, the chemical, but in particular the thermomechanical properties of such a molded article above 1550° C. do not meet the requirements typically encountered nowadays in gas turbines.
DE 26 24 299 A1 describes wearing parts for molten metal containers. The wearing parts are produced using a hydraulically setting refractory concrete with a high alumina content. The refractory concrete can contain a spinel-forming additive.
DE 24 59 601 B1 describes a refractory ceramic composition which contains carbon and silicon and to which spinel-forming oxides can be added.
EP 1 571 393 A1 describes a refractory material basically consisting of magnesium oxide and a considerable amount of a magnesium-containing spinel former. However, the handling and shaping of such a material must be classified as critical because of the hydration of the magnesium oxide to magnesium hydroxide.
Described in EP 1 820 586 A1 is a ceramic nozzle brick for use in or on a metallurgical vessel for receiving molten metal. It is formed wholly or partly from ceramic fibers, hollow spheres or foamed ceramic. Also described is a ceramic nozzle brick formed wholly or partly from at least 95% pure material of the group aluminum oxide, zirconium dioxide, preferably stabilized zirconium dioxide, magnesium oxide, calcium oxide, spinel.
In EP 0 535 233 B1, a monolithic refractory material of the aluminum oxide spinel type is described.
In WO 2005/031046 A1, spinel products with added binders are described.
Described in WO 01/60761 A1 is an annular insert for sliding plates, containing carbon and at least one material from the group MgO-sinter, MgO-spinel, caustic MgO.
Alternative approaches to using ceramic heat shield elements consist in using metal heat shield elements. Although metal heat shield elements are better able to withstand temperature fluctuations and mechanical stresses than ceramic heat shield elements, in gas turbine combustion chambers, for example, they require complex cooling of the heat shield, as they possess higher thermal conductivity than ceramic heat shield elements. Moreover, metal heat shield elements are more prone to corrosion and, because of their lower temperature stability, cannot be subjected to such high temperatures as ceramic heat shield elements.